ANN ARBOR, Mich.—The first national study of the prevalence of multiple partner fertility shows that 28 percent of all U.S. women with two or more children have children by more than one man.

The study will be presented April 1 in Washington, D.C., at the annual meeting of the Population Association of America.

"I was surprised at the prevalence," said demographer Cassandra Dorius, a postdoctoral fellow at the University of Michigan Institute for Social Research. "Multiple partner fertility is an important part of contemporary American family life, and a key component to the net of disadvantage that many poor and uneducated women face every day ."

While previous studies have examined how common multiple partner fertility is among younger women, or among women who live in urban areas, the research by Dorius is the first to assess prevalence among a national sample of U.S. women who have completed their child-bearing years.

Dorius analyzed data on nearly 4,000 U.S. women who were interviewed more than 20 times over a period of 27 years, as part of the National Longitudinal Survey of Youth. The data included detail on individual men in each household, capturing what demographers call "relationship churning." For nonresidential relationships, Dorius triangulated information from mother and child reports to establish common paternity.

She found that having children by different fathers was more common among minority women, with 59 percent of African American mothers, 35 percent of Hispanic mothers and 22 percent of white mothers with two or more children reporting multiple partner fertility. Women who were not living with a man when they gave birth and those with low income and less education were also more likely to have children by different men.

But she also found that multiple partner fertility is surprisingly common at all levels of income and education and is frequently tied to marriage and divorce rather than just single parenthood.

"I was a year into this project before I realized that my mother was one of these women," Dorius said. "We tend to think of women with multiple partner fertility as being only poor single women with little education and money, but in fact at some point, most were married, and working, and going to school, and doing all the things you're supposed to do to live the American Dream ."

Family researchers first began studying multiple partner fertility by exploring how men 'swap families' after having a child with a new partner, or reduce their financial support and physical involvement with nonresidential biological children when their ex-partners live with, or marry, someone new.

"Raising children who have different fathers is a major factor in the intergenerational transmission of disadvantage," Dorius said. "Juggling all the different needs and demands of fathers in at least two households, four or more pairs of grandparents, and two or more children creates a huge set of chronic stressors that families have to deal with for decades."

Dorius received support for the study from the National Science Foundation and the National Institute of Child Health and Development, and from the Center for Research on Diverse Family Contexts and the Joan Huber and William Form Research Fund.

Established in 1949, the University of Michigan Institute for Social Research (ISR) is the world's largest academic social science survey and research organization, and a world leader in developing and applying social science methodology, and in educating researchers and students from around the world. ISR conducts some of the most widely-cited studies in the nation, including the Thomson Reuters/University of Michigan Surveys of Consumers, the American National Election Studies, the Monitoring the Future Study, the Panel Study of Income Dynamics, the Health and Retirement Study, the Columbia County Longitudinal Study and the National Survey of Black Americans. ISR researchers also collaborate with social scientists in more than 60 nations on the World Values Surveys and other projects, and the Institute has established formal ties with universities in Poland, China, and South Africa. ISR is also home to the Inter-University Consortium for Political and Social Research (ICPSR), the world's largest digital social science data archive. Visit the ISR Web site at http://www.isr.umich.edu for more information.